The neonatal kidney develops most of its adult structural characteristics during childhood. Even when adult growth is complete, unilateral nephrectomy causes compensatory hypertrophy in the other kidney. Removal of over half the renal mass or administration of drugs may produce hyperplastic growth. Vascular parabiosis allows many of these systems to be turned on and off at will. These studies should help to ascertain the mechanisms of renal growth, permitting efficient and economical identification of the controlling agents and design of these agents for augmenting renal mass in patients with renal insufficiency. In addition, they should identify messenger RNAs activated in specific physiological processes. Although mRNA metabolism changes within an hour of unilateral nephrectomy, screening studies of total mRNA show no differences because the rare-copy mRNAs (95% of different mRNAs) are swamped by the mass of abundant-copy mRNAs (5% different mRNAs). By cloning rare-copy mRNAs and screening them by blot hybridization, changes in specific mRNAs during neonatal and compensatory growth will be identified. Rare-copy clones will not be identified by standard methods, however, but by hydrolyzing with the restriction enzyme Sau3A to permit greater efficiency and rapidity and to produce nucleotides of approximately 250 base-pairs, facilitating analysis of their sequences. Colony screening should allow identification of clones specific for kidney, of clones uniquely present in polysomal or nonpolysomal mRNP, and of quantitative changes in renal mRNAs during compensatory hypertrophy, neonatal growth, and metabolic perturbations, after loads of H+, K+, Na+, and of glutamine, for example. Specific cellular sites of origin of growth-regulated and developmentally-regulated mRNAs will be detected by in situ hybridization using probes labeled with deoxynucleotide thiotriphosphates to avoid problems posed by the vigorous peroxidase activity of kidney in conventional systems.